The association of proteins with small molecules and macromolecular targets is at the center of biology. The rates and mechanisms of these association processes constitute fundamental knowledge and provide avenues for designing therapeutic agents. The long-term objective of this project is the determination, via theory, simulation, and experiment, of association rates and mechanisms. Efforts in the pervious funding periods have culminated in the transient-complex theory. This computational methodology has allowed the protein association problem to be solved when the molecules are relatively rigid so that overall translational/rotational diffusion is rate- determining. The preset project aims to tackle the much more challenging problem where conformational change is closely coupled with the association process, and essential for biological function. The specific aims are the determination of rate constants and mechanisms for three classes of systems: (1) protein-protein association; (2) protein-ligand binding; and (3) Site-specific protein-DNA binding. The research will integrate computation and experiment, and bring methodological breakthroughs that enable realistic modeling of conformational changes in calculating protein association rate constants. The knowledge gained will shed light on important protein functions and offer broad mechanistic insights.